GATE CS 2014 Set 2 — Question 41

MCQ+2 / -0.67MediumSemaphoresProcess SynchronizationOperating SystemProducer-Consumer

Operating System → Process Synchronization → Producer-Consumer

Last updated 5 September 2026

Question

Consider the procedure below for the Producer-Consumer problem which uses semaphores: semaphore n = 0; semaphore s = 1; void producer() { while(true) { produce(); semWait(s); addToBuffer(); semSignal(s); semSignal(n); } } void consumer() { while(true) { semWait(s); semWait(n); removeFromBuffer(); semSignal(s); consume(); } } Which one of the following is TRUE ?

Correct answer (C) Deadlock occurs if the consumer succeeds in acquiring semaphore s when the buffer is empty. Solution The code shows a classic deadlock scenario in the Producer-Consumer problem due to incorrect ordering of semaphore operations in the consumer. 1. Initialization : n = 0 (items in buffer), s = 1 (mutex for buffer access). 2. Scenario : Suppose the buffer is empty (n = 0) and the consumer is scheduled first. 3. Consumer Execution : Executes semWait(s) : s becomes 0. Consumer acquires the lock on the buffer. Executes semWait(n) : Since n is 0, the consumer blocks (waits) for an item to be produced. 4. Deadlock : The consumer is blocked inside the critical section (holding s) waiting for n . The producer needs to run to increment n (via semSignal(n)). However, the producer first executes semWait(s) to acquire the buffer lock. Since s is 0 (held by the consumer), the producer blocks. Both processes are waiting for each other: Consumer waits for Producer to signal n, Producer waits for Consumer to signal s . Thus, deadlock occurs if the consumer acquires s when the buffer is empty. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (C) Deadlock occurs if the consumer succeeds in acquiring semaphore s when the buffer is empty. Solution The code shows a classic deadlock scenario in the Producer-Consumer problem due to incorrect ordering of semaphore operations in the consumer. 1. Initialization : n = 0 (items in buffer), s = 1 (mutex for buffer access). 2. Scenario : Suppose the buffer is empty (n = 0) and the consumer is scheduled first. 3. Consumer Execution : Executes semWait(s) : s becomes 0. Consumer acquires the lock on the buffer. Executes semWait(n) : Since n is 0, the consumer blocks (waits) for an item to be produced. 4. Deadlock : The consumer is blocked inside the critical section (holding s) waiting for n . The producer needs to run to increment n (via semSignal(n)). However, the producer first executes semWait(s) to acquire the buffer lock. Since s is 0 (held by the consumer), the producer blocks. Both processes are waiting for each other: Consumer waits for Producer to signal n, Producer waits for Consumer to signal s . Thus, deadlock occurs if the consumer acquires s when the buffer is empty.

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